Cardiac rhythm management (CRM) systems are a common solution for problems associated with the heart's inherent pacing capabilities. The fundamental components of a CRM system may include a pacemaker for creating electrical pulses to stimulate the heart and one or more electrodes for delivering the electrical pulses and sensing the heart's contraction in reaction to the stimulus. The heart's contraction in response to the electrical pulse is termed the evoked response.
Typically, the CRM system may monitor the heart for a set escape interval. An escape interval is a period of time during which the pacemaker will wait to send another electrical pulse to the heart. If this escape interval is exhausted without detection of a natural heart contraction, an electrical pulse may be delivered to the heart. Electrical pulses may be delivered at a set pacing stimulation frequency based on the duration of the escape interval.
Pacing abnormalities may occur in both the atrial and ventricular portions of the heart. For heart abnormalities, including such examples as total or partial heart block, arrhythmias, myocardial infarctions, congestive heart failure, congenital heart disorders, and various other problems, a pacing system for the ventricles may include one or more adaptive rate sensors. An adaptive rate sensor is a sensor that may function to monitor an individual's physical activity. If the adaptive rate sensor determines that additional cardiac output is desirable for the physiological requirements of an activity, the adaptive rate sensor may increase an adaptive-rate sensor indicated rate. The adaptive-rate sensor indicated rate is a rate provided by an adaptive rate sensor that may be used to decrease the duration of the escape interval of a pacemaker so that pacing stimulation frequency increases. As the pacing stimulation frequency is increased and the escape interval decreased, the heart is caused to beat at an increased rate and cardiac output may thereby be increased.
A prior art CRM system 100, including an adaptive rate sensor 110 coupled to a CRM device 130, is shown in FIG. 1. The adaptive rate sensor 110 may include an adaptive rate sensor or other such device or signal that functions to provide an adaptive-rate sensor indicated rate. The adaptive rate sensor 110 may communicate the adaptive-rate sensor indicated rate to the CRM device 130. The CRM device 130 may be a pacemaker or other such device that functions to pace the heart. The CRM device 130 may calculate an escape interval based on the adaptive-rate sensor indicated rate and deliver electrical pulses to the heart based on the escape interval.
Adaptive rate sensors may be used to detect a wide range of cardiac-output requirements and increase the pacing stimulation frequency according to the increased activity of the individual. Examples of activities that may necessitate an increase in pacing stimulation frequency are strenuous activities such as jogging or swimming, although any increase in activity may heighten an individual's cardiac output needs.
Presently there are three major types of commercial adaptive rate sensors available, including activity sensors, minute ventilation sensors, and QT interval sensors. All three types of sensors use different physiological criteria to measure changes in activity and therefore increased need for cardiac output. Unfortunately, all three sensor types may exhibit limitations in their use under various circumstances.
An activity sensor measures the acceleration of an individual by typically using an accelerometer. In situations such as, for example, when an individual goes from standing still to walking, the individual's need for cardiac output increases. The activity sensor measures an increase in acceleration as the individual walks and therefore increases the pacing stimulation frequency for the heart. This increase in the frequency of stimulation increases cardiac output. A limitation of this type of sensor is that the increase in pacing stimulation frequency is not always proportional to the increased workload for the individual, such as when the individual remains relatively stationary (and therefore the sensor records no change in acceleration) but experiences a heightened increase in the need for cardiac output. Examples of this situation include riding a stationary bike, where no acceleration occurs, and to a lesser extent, walking up stairs, where workload is not proportional to acceleration.
A minute ventilation sensor functions to monitor the breathing of the individual and equates increased respiration with the need for increased cardiac output. Specifically, the minute ventilation sensor can measure the volume of air inhaled and exhaled during a particular period of time, typically by measuring transthoracic impedance. An example of such a minute ventilation device can be found in U.S. Pat. No. 6,161,042 to Hartley et al. If the impedance-based minute ventilation sensor detects an increase in respiration, it assumes that there is an increase in the need for cardiac output and the sensor therefore increases the pacing stimulation frequency. While this type of sensor functions more proportionally to workload, a limitation of this sensor is the significant variation among individuals requiring individualized calibration. Another limitation of the minute ventilation sensor includes motion artifact, a phenomenon wherein certain movements by an individual, such as waving the individual's arms in the air, may be incorrectly interpreted by the impedance-based sensor as an increase in respiration.
A QT interval sensor functions to measure the interval between stimulus of the ventricle (Q) and the appearance of the T-wave signifying repolarization of the ventricle, as indicated on a typical electrocardiogram. A shortening of the QT interval may represent an increase in the need for cardiac output. Once again, a limitation of this type of sensor is a variation in QT intervals from individual to individual. For example, some individuals exhibit a QT interval that actually lengthens in duration during situations in which increased cardiac output is desirable. In addition, the QT interval sensor detects the interval from stimulation of the ventricle Q to detection of the T-wave, but contraction of the ventricle muscle actually occurs approximately 50 ms after stimulus. Therefore, in the relatively small QT interval of 120-250 ms, this lag between stimulus and contraction can have a significant impact on the accuracy of the measured rate of contraction.
A shortcoming common to all adaptive rate sensors is the failure to provide meaningful limitation on pacing stimulation frequency at high pacing rates. This failure to limit the pacing rate can lead to situations in which the pacing stimulation frequency exceeds the cardiac output needs of the individual. More importantly, the increase of pacing stimulation frequency beyond a certain threshold can actually result in a decrease, rather than increase, in cardiac output. This phenomenon may occur because the ventricle may be caused to contract at a rate that is faster than the ventricle can fill with blood.
Regarding activity sensors, because the rate provided by an activity sensor is not proportional to workload, there is no feedback provided to allow the activity sensor to immediately determine if the activity-sensor indicated rate exceeds the needs of the individual. Therefore, because no negative feedback exists, it is possible at high pacing stimulation frequencies for the activity sensor to provide adaptive-rate sensor indicated rates that exceed the individual's cardiac output needs.
A minute ventilation sensor may be proportional to workload and may exhibit some negative feedback characteristics. However, if a minute ventilation sensor is not calibrated appropriately, it too can indicate rates that exceed the needs of the individual, particularly if motion artifact is introduced. Further, motion artifact limitations are still presented in the minute ventilation sensor.
QT interval sensors do not have negative feedback in their normal operating range for adaptive-rate pacing. In fact, a QT interval sensor demonstrates a positive feedback characteristic because of the methodology utilized by the QT interval sensor to measure increased cardiac output need. This positive feedback is illustrated as follows:                (a) the QT interval sensor detects a decrease in the duration of the individual's QT interval and therefore increases the adaptive-rate sensor indicated rate;        (b) the increased adaptive-rate sensor indicated rate causes the pacemaker to decrease the duration of the escape interval, thereby causing an increase in the pacing stimulation frequency;        (c) the increase in pacing stimulation frequency causes the duration of the individual's QT interval to decrease; and        (d) the QT sensor detects this additional decrease in the duration of the individual's QT interval and therefore further increases the adaptive-rate sensor indicated rate.Therefore, positive feedback may occur in this situation where an increase in pacing stimulation frequency can cause a decrease in the duration of an individual's QT interval, thereby causing the QT interval sensor to further increase the adaptive-rate sensor indicated rate.        
The most sophisticated CRM systems implement a pair of the adaptive rate sensors described above (commonly activity and either minute ventilation or QT interval) to emphasize the strengths of particular sensors and lessen the limitations associated with each particular individual sensor. Such a system is the PULSAR™ MAX CRM system manufactured by Guidant Corporation, which utilizes an activity sensor in conjunction with a minute ventilation sensor. However, even in combination, the current adaptive rate sensors may have limitations, and therefore the potential exists for increased pacing stimulation frequencies that exceed an individual's cardiac output needs and may actually result in decreased cardiac output.
The adaptive-rate sensor indicated rate is defined herein to further include an intrinsic atrial rate as measured from the atria of the heart. An intrinsic atrial rate is typically utilized as a method of pacing the heart in individuals where natural conduction pathways between the atria and ventricles have been damaged, such as in left branch bundle block. In cases such as these, an electrode or other such sensor is placed in the atria to detect atrial contraction and communicate this contraction to the ventricles via a pacemaker or other device.
A common problem that may occur with use of intrinsic atrial pacing is atrial tachycardia, in which an individual's intrinsic atrial pacing rate reaches irregularly high rates. If the intrinsic atrial rate exceeds a certain threshold, use of the intrinsic atrial rate to pace the ventricles can cause the ventricles to contract at excessive rates. Therefore, use of the intrinsic atrial rate as a method of pacing the ventricles may also result in pacing stimulation frequencies that exceed an individual's cardiac output needs and may actually result in decreased cardiac output.